Autopsy results revealed he died from an accidental overdose of fentanyl – a drug 50 times more powerful than heroin.

According to the search warrants, authorities searched Paisley Park as well as looking through the mobile phone records of Prince's associates, and the star’s email accounts to try to determine how he got the drug.

Officers found Prince's Paisley Park home was littered with prescription drugs

Dr Michael Todd Schulenberg, who saw Prince April 7, 2016, and again on April 20, reportedly admitted to prescribing the drug the same day as the emergency plane landing "but put the prescription in Kirk Johnson's name for Prince's privacy".

Authorities also searched Johnson's mobile phone records, to see who he was communicating with in the month before Prince died.

Messages left with attorneys for Schulenberg and Johnson weren't immediately returned Monday.

Fans left tributes outside Prince's Minnesota home after his death last year

Schulenberg has an active medical license and is currently practicing medicine in Minnesota.

His attorney, Amy Conners, told the AP last week there are no restrictions on his license.

Investigators haven't interviewed either Johnson or Schulenberg since the hours after Prince died, an official with knowledge of the investigation told AP on condition of anonymity because the investigation is ongoing.

The documents were unsealed in Carver County District Court in Minnesota as the year-long probe into Prince's death continues.